1. Field of the Invention
This invention relates generally to a method for preventing gas pressure in a pressure vessel from dropping below a minimum allowable pressure and, more particularly, to a method for preventing pressure in one or more pressure vessels in a hydrogen storage system from dropping below a minimum allowable pressure which monitors the normal tolerance-driven fluctuations in pressure readings at a pressure sensor downstream of a pressure regulator and, if a pressure drop in excess of the normal fluctuations is detected, shuts down the hydrogen storage system to prevent the pressure in the vessels from dropping too low.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
Typically hydrogen gas is stored at high pressure in pressure vessels on the vehicle to provide the hydrogen necessary for the fuel cell system. The pressure in the vessels can be 700 bar or more. In one known design, the pressure vessels include an inner plastic liner that provides a gas tight seal for the hydrogen gas, and an outer carbon fiber composite layer that provides the structural integrity of the vessel. A hydrogen storage system typically includes at least one pressure regulator that reduces the pressure of the hydrogen gas from the high pressure of the vessels to a pressure suitable for the fuel cell system.
If the pressure within the vessels falls below a certain value, and the vessels are then refilled at a high rate of pressure increase, the inner liner layer may begin to separate from the outer structural layer. This separation could cause inner liner damage and loss of leak-tightness, and thus must be avoided. A current solution to avoid this separation is to maintain a high enough pressure within the vessels to prevent inner liner layer separation. For one exemplary vessel design, a minimum pressure of 20 bar must be maintained in the vessels to prevent the inner liner layer from separating from the outer structural layer.
One or more pressure sensors provide a measurement of the hydrogen gas pressure within the vessels and elsewhere in the hydrogen storage system. Because the pressure sensors employed in these types of systems need to provide a pressure measurement over a range of nearly 1000 bar, and they need to be relatively inexpensive, they typically have a tolerance band of about 1.5%, which gives an accuracy of +/−15 bar. Further, considering the measurement requirements of the sensor wiring over the entire temperature range that the vessels may encounter typically provides a measurement accuracy of +/−35 bar, which is added to the 20 bar minimum allowable pressure to provide the desired safety margin. Thus, in typical system designs, hydrogen discharge from the vessels needs to be stopped at a vessel pressure sensor reading of about 55 bar, resulting in about 5% of the hydrogen gas within the vessels not being usable for vehicle operation.
A method is needed for reliably protecting the pressure vessels from dropping below the minimum allowable pressure, but still allowing the most possible hydrogen gas to be consumed by the fuel cell. Such a method would allow the vehicle to be driven a greater distance between refueling events, thus improving customer satisfaction, while still protecting the vessels from dropping below the minimum allowable pressure.